Motor control circuit



June 7, 1949. B. OSTENDORF, JR 2,472,611

MOTOR CONTROL CIRCUIT.

Filed June 2, 1945 .g INVENTOR B. OSTE'NOORF, JR.

A TTORNE V Patented June 7, 1949 2,472,611 MOTOR CONTROL CIRCUIT Bernard Ostendorf, Jr., Jamaica, N. Y., assignmto Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 2, 1945, Serial No. 597,205 2 Claims. (01. 318-257) This invention relates to an amplifier and more particularly to a balanced amplifier responsive to a control signal for controlling the operation of a servomotor.

It is an object of the invention to provide a servo-amplifier in which the tendency of the servomotor to hunt upon sudden changes in the control signal is materially reduced.

A further object of the invention is to provide means in a servo-amplifier for slowing down the effect upon the servomotor of sudden large voltage changes which would tend to cause the motor to accelerate too rapidly with a consequent shock to apparatus driven thereby, but which means does not respond to sudden voltage changes which would tend to cause the motor to decelerate rapidly.

The servo-amplifier in accordance with the present invention comprises an input filter section, a direct current amplifier section, an equalizer section, a clutch section, a multivibrator section, a bufier section and a relay section.

One feature of the invention by which the objects thereof are attained comprises two triode equalizer tubes which may; for convenience, be a twin tube. The input filter connects through series resistors to the first direct current amplifier stage. The two equalizer tubes are resistance-condenser coupled and are cross-connected across the aforementioned series resistors. This connection is such that any changes in the direct current voltage going from the filter to the direct current amplifier stage is aided by the condenser coupled equalizer stage. The equalizer tubes respond only to voltage changes and accordingly have no effect on the servo-amplifier if there is no change in the amplitude of the direct current component of the control signal. Any sudden change in the direct current component actuates the equalizer tubes in a manner which steps up the rate of acceleration or deceleration and thus acts as a brake to prevent theservomotor from hunting.

Another feature of the invention comprises two diode tubes having their plates connected respectively to the output plates of the last stage of the amplifier section and having their cathodes reversely connected through condensers to the same amplifier outputs. Preferably these tubes are two units of a twin diode tube. The clutch tubes slow down the effect of sudden large voltage changes which would tend to rotate the motor but do not affect voltage changes which would result in stopping the rotation of the motor. A sudden large signal voltage may be presented to the servo-amplifierwhen the amplifier is switched into service after a period of idleness or if a control wheel in the transmitter to which the servo-amplifier is switched is jerked rather than movedat a uniform rate of speed. T e w i d u h ube tem o a y hort:

circuits any incoming signal voltage output from the preceding amplifier stage. This condition lasts only during the brief period While the condenser is charging. Thus the tube tends to moderate extremely large signal voltages by presenting them somewhat more gradually to the multivibrator tube. Due to the polarity relations of the circuit only one unit of the clutch tube conducts at any instant.

A further feature of the invention is the provision of a twin triode multivibrator tube which receives the outputs from the amplifier. It has symmetrical feedback circuits for each of its two sections. Under balanced operating conditions during which both grid bias voltages are equal, the multivibrator oscillates to produce approximately balanced square waves at its plate terminals, the waves alternating at a frequency of about 40 cycles per second with approximately equal open and closed periods, the output from one plate is the negative component of the output of the other plate. However, if the variable direct current signal voltages impressed on the two grid terminals become unequal, then the periods of the square wave will also become unequal. The square wave outputs from the two plates of the multivibrator tube are employed to drive two relays which in turn apply driving impulses to the armature circuit of the servomotor.

Other features of the invention will become apparent upon reference to the following detailed description whenread in connection with the single sheet of drawing.

The servo-amplifier is responsive to a nongrounded control signal or error voltage, consisting of rectified half sine waves, connected between terminals 4 and 2. The error voltages are positive or negative only with respect to each other. A positive voltage on terminal 1 with respect to terminal 2 can exist or conversely a positive voltage on terminalZ with respect to terminal I can occur; the input resistors RH and RI 8 provide a neutral orcenter tap voltage which is connected to circuit zero or neutral. The polarity of the input voltage determines whether the motor M will rotate in a clockwise or counterclockwise direction and the amplitude of the voltage determines the relative speed at which the motor will run.

The pulsating voltage appliedbetween the terminals l and 2 is filtered by the low-pass lattice type filter comprising the series connected choke coils LI and L2, L3'and L4 and bridged condensers CI to C5, inclusive. The impedance of the filter matches the load impedance of 39,000 ohms (resistors R26 and R48) connected to the grid terminals of the twin triode amplifier tube VTS.

The double resistor-double capacitor bridge consisting of resistors RH and RIB and con densers CIA and C B balances out any spurious alternating or direct current voltages leaking to tive on one terminal as compared with the opposite polarity on the other terminal of the pair.

The double condenser" bridge comprising condensers Cd and C5 with its center tap also connected to the -V reference bus bar of the amplifier, balances out any extraneous longitudinal voltage which may be present at the signal terminals l and 2.

The twin triode equalizer tube VT? has its control electrodes connected to the output signal conductors 3 and t of the filter section. of the amplifier through the condensers C20 and Cid, respectively, and through the resistors R22 and RM with the 0-V reference bus bar. Tube VT? also has its cathodes connected through the resistances RM and R43 to the 0-V reference bus bar. Plate potentials for the tube VT? are supplied through the voltage dividers comprising resistors R23 and R20, of 27,000 and 120,000 ohms respectively, for the plate of the upper unit and resistors R and R00, of 27,000 and 120,000 ohms respectively, for the plate of the lower unit. One end of each of these voltage dividers is connected to the 0-V bus bar and the other end of each divider is connected to the 150-V bus bar. Any 400-cycle alternating current hum which is superimposed on the output of tube VT! is thus reduced in signal strength as regards the control grid of the succeeding amplifier tube VTB in the ratio of 27,000 ohms to 147,000 ohms. This reduction in alternating current hum is accomplished without sacrificing gain in the equalizer tube VT'l. The plates of tube VTl are connected with the grids of the succeeding amplifier tube VTt through the condensers C20 and C25.

The relative importance of the equalizer tube can be better understood if it is realized that the servo-amplifier could be operated with the tube removed from the socket. However, the tube adds very desirable operating features previously mentioned, namely that of increasing the effect of signal voltage changes. This is of particular importance in connection with a signal voltage change resulting from stopping or changing the direction of the control element of an associated transmitter. A quick effective braking of the control circuit helps to prevent the motor and i the follow-up apparatus which may be driven by the motor from overrunning or overshooting.

The necessity for servo accuracy requires the use of a high gain amplifier which, of course, tends to offer the basic condition for producing electro-mechanical oscillations. In the event of a tendency to oscillate, the equalizer tube VTl enables the mechanism to more rapidly recognize a change in the rate of motion. This tends to prevent it from overshooting through zero, then reversing its direction and coasting back past Zero and continuing to do this thus setting up a hunt or oscillation.

Tube VT'll provides a class Aamplifier having a rising gain versus frequency characteristic. The voltage gain is zero at zerocycles, about 2 at 5 cycles and rises to at frequencies higher than 4 cycles. Accordingly there is no circuit function performed by the tube if the transmitter is operated at a slow or at a very gradually accelerated rate.

It will be assumed that a positive control signal changing in a positive direction exists on terminal i with respect to terminal 2. A positive direct current bias is placed on the control grid of the lower unit of tube VTO whereas a negative voltage is impressed on the control grid of the lower unit of tube VTl. The output of the lower unit of tube VT'i is therefore in a positive direction. This positive voltage is connected to the control grid of the lower unit of tube VT8 to reenforce the signal voltage rather than to oppose it. It can readily be shown that a similar correspondence in polarity exists between the signal voltage and the output voltage of the upper unit of tube VT? and that the voltages join to bias the grid of the upper unit of amplifier tube VTS.

It can be stated therefore that a change in signal voltage produces a change in direct current component which places the equalizer tube VT? in operation to magnify the signal voltage change connected to the succeeding amplifier stage.

The dual triode amplifier tubes VTil and VTE? provide a two-stage high gain class A directcurrent amplifier. Tube VTO has an amplication factor of about 70 to l and employs a high plate load resistance comprising resistors R28 and R50. A condenser-resistance filter is provided in the input leads of tube VTil to reduce further any residual im-cycle alternating current hum which may have passed the amplifier tube VTO. These filters comprise the resistor-condenser combinations R20C2i and REM-C20.

The cathode resistor network associated with the tube VTB produces a bias of about 1.5 volts and includes a screwdriver-adjustable potentiometer R68 to permit the balancing of the tube operation by offsetting resistance variations of the apparatus. The resistance network includes the potentiometer R68 connected between the cathodes of the two units of tube VTB connected in parallel with the series-connected equal resistances R00 and R67, the junction point between which is connected to the slider of potentiometer R68 and through resistance R55 to the +150--V bus bar, and resistors R and R49 which connect the cathodes of the tube to the 0--V bus bar.

The cathode resistor R which is common to the two cathodes of the twin triode amplifier tube VT9 provides stabilized bias for such tube. Tube VT9 is directly coupled to the output of tube VTB through the coupling resistors R29 and REI and is connected in push-pull circuit to reduce longitudinal voltages arising from the variations in the preceding stages.

As in the case of the equalizer tube VTl, the servo-amplifier would operate even if the clutch tube Vl0 were removed from its socket. However, the tube slows down the effect of those sud den large voltage changes which tend to cause the motor to rotate but does not effect changes which result in stopping the rotation of the motor. A sudden large signal voltage may be pre-- sented to the servo-amplifier when the servoamplifier is switched into connection with the control transmitter after a period of idleness or if a handwheel of the transmitter is jerked rather than moved at a uniform rate of speed. The clutch tube, a twin diode, temporarily short-circuits any incoming positive signal voltage. This condition lasts only during the brief period while either condenser C22 or condenser C21 is charg ing. Thus the tube tends to moderate extremely large signal voltages by presenting themsomewhat more gradually to the multivibrator tube but it introduces no delay in braking the circuit.

Amplifier tube VT9 is directly coupled to the multivibrator tube VTI l. The coupling between the upper units of these tubes comprises the voltage divider including resistors R3l, R33, R35 and R36 connected in series between the +450-V bus bar and the V bus bar, theplate of the upper unit of tube VTB being connected to the junction point between resistors R3l and R33 and the grid of tube VTH being connected to the junction point between resistors R35 and R36. The coupling between the lower units of these tubes comprises the voltage divider including resistors R53, R55, R51 and R58 connected in series between the +450-V bus bar and the 0-V bus bar, the plate of the lower unit of tube VT9 being connected to the junction point between resistors R53 and R55 and the grid of tube VTll being connected to the junction point between resistors R51 and R58.

The twin diode clutch tube VTI!) shunts only those voltage outputs of the amplifier tube VTS which change in a positive direction. Due to the polarity relations of the circuit'only one unit of tube VTIU conducts at any instant. To accomplish this the plates of amplifier tube VT 9 are interconnected through the two resistors R32 and R54, the junction point between which resistors is connected through other resistors R34 and R56 with the cathodes of tube VT); The cathode of the upper unit of tube VTIII is also connected through the condenser C22 to the junction point between resistors R33 and R35 and the cathode of the lower unit of tube VTIO is connected through condenser 021 to the junction point between resistors R55 and R51. The plate of the upper unit of tube VTID is connected to the plate of the lower unit of tube W9 and the plate of the lower unit of tube VTIO is connected to the plate of the upper unit VTB. The shunted positive voltage produced in one unit of the tube VTIB is superimposed on one grid of the multivibrator tube VTII but in a direction opposite to that of the normal signal output from the amplifier tube VT9 and accordingly decreases the net difference otherwise existing between the two grids of the multivibrator tube VTI l.

Assuming that a signal voltage at a particular instant is changing in a positive direction on the terminal I, while it is changing correspondingly in a negative direction on terminal 2, then the polarity relations of the voltage conditions in the servo-amplifier circuit are as shown in the following table:

It may be noted from the above table that the output voltages on the plates of the multivibrator tube VIll will differ in peak values by amounts which are less than would result from the effect of signal voltages alone due to the action of the clutch tube VTI 0. In other words the unbalance of the circuit is temporarily lessened. The shunting action of tube VTIB in temporarily reducing this voltage difference is effective in slowing down only sudden changes in voltage which would cause the motorto speed up. From its rectifying nature the tube has no effect on voltage changes in a negative direction which occur when braking the circuit for stopping the motor.

The dual multivibrator tube VTII has symmetrical feedback circuits for each of its two units. Under balanced operating conditions during which both grid bias voltages are equal, the multivibrator oscillates to produce approximately balanced square waves at its plate terminals, the waves alternating at a frequency of about 40 cycles per second with approximately equal open and closed periods. The outputfrom one plate is the inverted counterpart of the output from the other plate. This relationship results from the symmetrical circuit conditions wherein the frequency of oscillation and the rela tion of the periods are established by the constants of the two resistor-condenser feedback circuits. One of these circuits includes the resistor R3! connected in series with condenser C28 between the plate of the upper unit of the tube and the grid of the lower unit. The other of these circuits includes the resistor R59 connected in 1 series with condenser C23 between the plate and the lower unit of the tube and the grid of the upper unit. As before stated, the grids of the tube are also connected through resistors to the plates of the amplifier tube VT9.

The cathodes of the tube VTI l are connected to the +150--V bus bar and plate potential from the +l50V bus bar is supplied to the plate of the upper unit through serially connected resistances R3! and R38 and to the plate of the lower unit through serially connected resistances R59 and R60.

If the signal voltages impressed on the two grids of tube VTH should become unequal, then the periods of the square waves would become unequal. The time at which one unit of the multivibrator tube cuts off or again conducts then depends not only upon the resistor-condenser time constant of the circuit but also on the level of potential toward which the circuit tends to charge. For example, with a given resistor-condenser circuit and a fixed starting voltage, it will take a longer time for the circuit to charge to a potential resulting in tube conduction if the charging cycle approaches a lower rather the. a higher voltage on the grid.

It is to be understood that the various components of the servo-amplifier are balanced under operating conditions only when the servomotor operated apparatus and the transmitter are oriented in exactly the same directionsince then no error or signal voltage is impressed across the terminals l and 2. At all other times unbalanced voltages occur, the polarities and magnitudes of which vary depending upon the direction and amount that the transmitter and motor-operated apparatus are out of phase. If unbalanced voltages are eventually connected to the grids of the two units of multivibrator tube VTI I, the unbalanced voltages cause one unit to take a longer time to out in than the other, thus producing un balanced square waves as an output. The progression of the unbalanced period of the waves in a positive or negative direction depends upon the direction in which the motor driven apparatus departs from the transmitter and. the relative duration of the periods are approximately proportional to the magnitude of the out of phase relationship (error).

The multivibrator tube VT H is coupled with the dual triode buffer tube VTIZ which functions as a power amplifier. The plate of the upper unit of tube VTH is coupled with the grid of the upper unit of tube VTl2 through resistor R39 which is included in the voltage divider extending from the +450-V bus bar through the serially connected resistors R38, R31, R39 and R40 to the -V bus bar. The plate of the lower unit of tube VTl i is coupled with the grid of the lower unit of tube VTlZ through resistor RBI which is included in the voltage divider extending from the +i50V bus bar through the serially connected resistors R6, R59, Rtl and R62 to the 0-V bus bar.

The two outputs of the buffer tube VTI2 operate two control relays which jointly cause the motor M to rotate in a, clockwise or a counterclockwise direction and at varying speeds. The plate of the upper unit of tube VTlZ is connected through resistor RM and through the winding of relay A to the +450V bus bar and the plate of the lower unit of the tube VTlZ is connected through resistor R63 and through the Winding of relay B to the +450-V bus bar. Condenser C62 provides a shunt across the winding of relay A to absorb the inductive disturbances arising from the operation and the release of the relay and condenser C63 performs a similar function in connection with relay B. An important function of the buffer tube is to separate the relay electrically from the multivibrator circuit so that inductive disturbances arising from the operation of the relays will not produce adverse effects in the multivibrator operation.

The relays A and B are preferably of the mercury contact type, such as is disclosed in the application of E. T. Burton, Serial No. 545,985, filed July 21, 1944, now Patent No. 2,459,306 patented January 18, 1944, but relays of other types could be used, provided they are fast in operation, sensitive and are capable of handling a considerable power load Over their contacts. Each of these relays comprises an envelope of glass or other suitable material, through the bottom of which an armature terminal is sealed and through the top of which two other terminals are sealed. Secured to the inner end of one of the upper terminals is a front contact of magnetic material and secured to the inner end of the other upper terminal is a back contact of non-magnetic material. Secured to the lower terminal by a reed is an armature of magnetic material biased against the back contact and attractable toward the front contact. A pool of mercury is placed in the bottom of the envelope from which mercury is conducted to the contacts by wick action. Surrounding the envelope is an operating coil or winding which when energized sets up a flow of magnetic flux through the armature and front contact to cause the movement of the armature towards the front contact.

The motor M has its field F connected across the terminals 5 and t to which a source of 24- volt direct current is connected. The armature circuit of the motor is connected between the swing contacts of the relays through the choke coil L15, paralleled by resistance R72, to limit the alternating components of the current in the motor armature and to serve to smooth the operation of the motor. The choke coil Lit, paralleled by resistor R11, is included in the connection from terminal 5 of the direct current source to the-front contacts of the relays to minimise contact deterioration durin the brief periods when a contact transfer occurs in either relay and all the contacts are in engagement. A network of resistors and condensers is provided for protecting the contacts of the relays. For example the swing and back contact of relay A are bridged by the series combination of resistor R69 and condenser 039A and also by a bridge consisting of resistor R13 in series with a parallel combination involving resistor R1 and condenser G4 I. These bridges are separated by an inductive resistor RM. Similar networks are provided for protecting the swing and front contact of relay A, the swing and back contact of relay l3 and the swing and front contact of relay B.

The association of the two control relays A and B with the multivibrator and buffer tube previously described, provides an arrangement whereby one relay is operated while the other is released. The relays operate in this manner at a rate of about 40 cycles per second. If the servo-amplifier is in a balanced condition, that is, no signal voltage is applied across the terminals l and 2, symmetrical square waves are produced at the outputs of the buffer tube VTIZ and the relays A and B alternately operate and release in response to the essentially square waves but in an opposite order.

Accordingly with relay A operated and relay B released, current will flow from terminal 5 of the power source, through the resistor R'H in parallel with choke coil Lit, through resistor R75, over the front contact of relay A, through resistor B72 in parallel with choke coil Lib, through the armature circuit of motor M, over the back contact of relay B and through resistor R16 to the terminal 6 of the power source. When relay B is operated and relay A is released, current will flow from terminal 5 of the power source, through resistor Rll in parallel with choke coil Ll3, through resistor R'i'l, over the front contact of relay B, through the armature circuit of motor M, through the resistor RTE in parallel with choke coil LIE, over the back contact of relay A and through resistor RM to the terminal ii of the power source. These impulses of power are equal and are transmitted alternately in opposite directions through the armature circuit of the motor and accordingly the net direct current voltage connected across the armature circuit of the motor is zero and the motor remains at rest. However, the armature will vibrate sufficiently to overcome the static friction of the motor enabling the motor to rotate smoothly at very low rates of speed.

If unbalanced voltage conditions exist, resulting for example in relay A remaining in its front contact closure position for a longer time in any given interval than it remains in its back contact closure position and at the same time, relay B remains in its back contact closure position for a longer time than it remains in its front contact closure position, the impulse of power transmitted through the armature circuit of the motor M, when relay A is operated and relay B is released, will be lengthened and the impulse of power transmitted in the opposite direction through the armature circuit of the motor when relay A is released and relayB is operated will be shortened and the net voltage across the armature circuit of the motor will cause the motor to turn in one direction of rotation. This net effective voltage will, of course, govern the speed of rotation. A higher net voltage results if the closed period for operating one of the relays greatly exceeds in duration the open period during which the relay is released.

Power for operating the tubes of the servoamplifier is provided through the equipment shown in the lower left portion of the drawing. This power is derived from a source of 400-cycle l15-volt alternating current connected across terminals 1 and 8 which are connected to the terminals of the primary winding Pi of power transformer Ti. Heater supply current is applied to the filaments of tubes VTI, VTB, VT9, VTH and VTIZ from the secondary winding Si of transformer TI and heater supply current is applied through the filament of tube VTID from the secondary winding S2 of transformer Tl.

For supplying +300 volts between the +450-V and +150V bus bars, the full wave rectifier tube VTI is provided, the filament of which is heated by current from the secondary winding S3 of the transformer TI. The secondary winding S4 of transformer Tl supplies 400-cycle, 400-V01t alternating current to the plates of tubes VTI, the mid-point terminal of the winding S4 being connected to the +150-V bus bar.

The rectified output of tube VTI is smoothed by a filter consisting of the series choke coil L9 and the bridged condensers CH and CIZ and is regulated at a potential of about +300 volts with respect to the +150--V bus bar connected to the center tap of the secondary winding S4 of the transformer TI. This regulation is accomplished by vacuum tubes VT3 and VT4. The pentode tube VT3 serves as a variable impedance connected in series between the output of tube VTI and the +450-V bus bar. The value of the impedance in series with the output depends upon the plate-cathode current of tube VT3 which in turn is changed by the potential connected to the control grid of the same tube. The control grid is influenced by the voltage drop across resistor R2, a drop which is caused by the plate-cathode current of the amplifier pentode tube VT4.

The plate-cathode current of tube VT i changes depending on variations in the potential connected to the control grid of this tube. A varying grid potential is obtained from a point on a voltage divider bridge comprising resistors R1 and R8 connected between the +450-V and the V bus bars.

If the voltage on the bus bar designated +450-V should tend to rise, a proportion of the voltage difierence between this point of 0 volts is impressed on the control grid of tube VT4 and results in an increased plate-cathode current and accordingly an increased voltage drop across the resistor R2. This increased voltage drop lowers the potential on the control grid of the regulator tube VT3 thereby reducing its plate output. The reduction in plate output is, of course, accomplished by an increase in the impedance between the plate and cathode of tube VT3, thus tending to lower the voltage on the +450V bus bar. All of these actions to bring the output into regulation occur almost instantaneously.

In a similar manner a tendency of the voltage between thebus bars +450-V and 0-V to decrease, causes a decrease of the potential applied to the control grid of tube VT4 which results in a decrease in the plate-cathode current and accordingly a decreased voltage drop through resistor R2. This decreased voltage drop increases the potential on the control grid of the regulator tube VT3 thereby increasing its plate output. The increase in plate output is accomplished by a decrease in the impedance between the plate and cathode of tube VT3, thus tending to raise the voltage on the +450V bus bar. As stated previously, all of these actions take place almost instantaneously.

The junction of the two resistors R9 and Bit bridged in series between the +450-V and +V bus bars is connected to the filaments of tubes VT3, VT l and VTIU to keep the potential of these tubes within the heater cathode requirement. Anti-sing resistors for parasitic suppression include the screen grid resistor RI and the grid resistor R2 associated with tube VT3.

For supplying 150 volts between the +150-V and 0V bus bars, a half wave rectifier tube VT2 is provided. A LOO-volt, 400-cyc1e alternating current input to the tube is obtained from the left half of the secondary winding S4 of transformer TI and the filament is heated from the secondary winding S5 of the transformer. The direct current output from the plate of tube VT2 is smoothed by a double section filter comprising the series choke coils LID and LI l and the bridged condensers CIA, C15, CIS and CH. The rectified voltage output is reduced by the series resistor R4 and the output potential is fixed at 150 volts by the shunt regulator gas tube VT5.

What is claimed is:

1. An amplifying system comprising two power amplifier tubes, signal voltage amplifier tubes arranged in push-pull relationship for applying an amplified signal potential to the control grids of said power amplifier tubes, two condensers, a

rectifier tube in the charging circuit of each of said condensers responsive to a sudden increase of said amplified signal potential in a positive direction for applying a potential to the control grid of one of said power amplifier tubes in opposition to said amplified signal potential but unresponsive to a sudden decrease of said amplified signal potential, a signal input circuit for applying equal signal potentials of opposite polarity upon the inputs of said signal voltage amplifier tubes, two condensers chargeable in response to changes in the signal input potentials, and equalizer tubes responsive to the charging of said latter condensers for applying amplified derivatives of said input potentials to the inputs of said voltage amplifier tubes to increase the effects of said signal potentials upon said voltage amplifier tubes.

2. In a servo-amplifier system, a multivibrator comprising two tubes with substantially identical condenser resistance feedback circuits whereby with balanced grid potentials applied to the grids of said tubes oppositely poled square waves are generated in the plate circuits of said tubes having substantially equal closed and open periods, power amplifier tubes upon the inputs of which said output waves are impressed, relays in the output circuits of said power amplifier tubes operable in alternation in response to said amplified output waves, a source of power, a servo-motor operable in response to impulses transmitted thereto from said source by the alternate operation of said relays, a signal input circuit, and means responsive to the application of a signal potential to said signal input circuit to render the grid potentials of said multivibrator tubes unequal whereby the closed periods of one of said output waves become lengthened and the closed periods of the other of said output waves become shortened to increase the net voltage of the impulses transmitted by said relays to said motor whereby said motor is caused to run in a direction 1() determined by the polarity of the net voltage and at a speed determined by the magnitude of the net voltage.

BERNARD OSTENDORF, JR.

, REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,315,539 Carson Sept. 9, 1919 1,800,303 Lindsay Apr. 14, 1931 2,099,536 Scherbatskoy et a1. Nov. 16, 1937 2,244,695 Hathaway June 10, 1941 2,286,106 Ritzman June 9, 1942 2,437,951 Godet Mar. 16, 1948 

